I have a nano 4th generation, now I am waiting to receive via ebay a nano 7th generation.
I will want to put my music contained in my itunes prog. to the "new " ipod.
Do I just plug it in to my pc? and will itunes except it and sync to it? and forget the old unit.
Any advice would be useful.

It should be as simple as that. Yes. You will probably have to press the Sync button as the default is not to automatically sync when connected (but you can change this within the device settings in iTunes).

It won't forget the old unit. It will simply add the new iPod as a new device.